PROJECT SUMMARY The term scleroderma encompasses a diverse set of conditions that are unified by excessive collagen deposition in the skin and often internal organs. A common and severe but genetically intractable form called systemic sclerosis (SSc) shows adult-onset fibrosis in association with autoantibody production. Recently, we showed that a Mendelian form of scleroderma called stiff skin syndrome (SSS) is caused by mutations in the integrin-binding domain of fibrillin-1, a matrix protein that also regulates the activity of the profibrotic cytokine TGF?. These mutations impair the ability of cells to make contact with fibrillin-1 via bridging interactions with integrins. Knock-in mouse models of SSS show fully-penetrant dermal fibrosis and all of the autoimmune abnormalities characteristic of SSc in association with tissue infiltration of activated plasmacytoid dendritic cells (pDCs) that are capable of orchestrating immunologic dysregulation and fibrosis through the production of inflammatory cytokines such as IFN-? and IL6. Fibrosis and autoinflammation are prevented (or even reversed) in SSS mice with interventions that normalize the abnormally high expression of ?v?3 integrin by pDCs; this complete rescue is phenocopied in SSS mice by blocking cellular signaling pathways known to influence pDC performance (e.g. TGF? and ERK1/2). Notably, fibroblasts derived from patients with diffuse and active SSc also show concordant phenotypes and therapeutic responses. The work proposed in this application will fully exploit the first bona fide mouse model of a human presentation of scleroderma. Aim 1 will interrogate the relative contributions of perturbations in adaptive and innate immunity by building upon our exciting preliminary data showing full maintenance or complete abrogation of dermal fibrosis in SSS mice upon introduction of the complete null state for Rag2 (a factor required for the production of T and B cells) or conditional depletion of pDCs, respectively. We will also explore the relevance of a potential axis for matrix-initiated pDC activation that has been descriptively implicated in other autoimmune disorders and is now supported by our preliminary data for SSS. Aim 2 will assess the therapeutic potential of a pharmacologic antagonist of ?v?3 integrin in scleroderma that is already in clinical development for cancer. It will also comprehensively scrutinize the role of specific pDC effector functions including the production of IFN-?, IL6, or CXCL4 (a factor that shows excellent temporal correlation with the onset, severity and progression of SSc). Aim 3 will build upon prior work and our preliminary data showing that TGF?-induced transition of diverse cell types to invasive collagen-producing myofibroblasts is critically dependent upon Wnt activity and the CAPN9/S2 dimeric calpain complex, respectively. These studies have the strong potential to unveil novel therapeutic strategies for SSS, SSc and perhaps more common presentations of fibrosis.